Back to EveryPatent.com
United States Patent |
5,757,244
|
Nonaka
,   et al.
|
May 26, 1998
|
Digital control type oscillation circuit of portable telephone, crystal
resonator oscillation frequency calculating method, and outputfrequency
correcting method
Abstract
In a portable telephone which uses a CPU, a memory, a temperature sensor, a
D/A converter, and an A/d converter as control elements, and which has an
oscillation circuit including a crystal resonator and a
variable-capacitance diode, temperature compensation of the output
frequency is made by using these existing control elements. The memory is
previously stored with control information for correcting an output
frequency drift of the portable telephone caused by a temperature change.
The temperature of the oscillation circuit is detected with the
temperature sensor and converted into a digital value in the A/D
converter. The CPU reads control information corresponding to the detected
temperature from the memory and applies it to the variable-capacitance
diode of the oscillation circuit through the D/A converter, thereby
maintaining the output frequency at a constant level. Also disclosed is a
method of efficiently calculating an oscillation frequency of a crystal
resonator with a reduced number of points of measurement for a temperature
characteristic of the crystal resonator. A method of accurately carrying
out the above-described temperature compensation is also disclosed.
Inventors:
|
Nonaka; Youji (Yokohama, JP);
Ishida; Yuji (Yokohama, JP);
Suwa; Youji (Yokohama, JP);
Hiramoto; Toshikazu (Yokohama, JP)
|
Assignee:
|
Kyocera Corporation (Kyoto, JP)
|
Appl. No.:
|
606072 |
Filed:
|
February 23, 1996 |
Current U.S. Class: |
331/176; 310/315 |
Intern'l Class: |
H03L 001/02 |
Field of Search: |
331/176,158
310/315,346
|
References Cited
U.S. Patent Documents
4254382 | Mar., 1981 | Keller et al. | 331/116.
|
4922212 | May., 1990 | Roberts et al. | 331/176.
|
5117206 | May., 1992 | Imamura | 331/158.
|
5392005 | Feb., 1995 | Bortolini et al. | 331/44.
|
5548252 | Aug., 1996 | Watanabe et al. | 332/176.
|
Foreign Patent Documents |
58-184809 | Oct., 1983 | JP.
| |
Primary Examiner: Callahan; Timothy P.
Assistant Examiner: Luu; An T.
Attorney, Agent or Firm: Loeb & Loeb LLP
Claims
What we claim is:
1. A method of calculating an oscillation frequency of a crystal resonator
by obtaining each coefficient of a temperature characteristic of the
crystal resonator which is expressed by a polynomial of degree n
(n.gtoreq.3) with respect to temperature, the method comprising:
using a mean value as m (1.ltoreq.m<n) coefficient among n coefficients of
the polynomial, and
determining the remaining n-m coefficients and a constant from n-m+l
temperature points and frequency data measured at the temperature points,
thereby calculating an oscillation frequency.
2. The method of claim 1, wherein the polynomial of degree n is
approximated by a cubic expression having a constant and first, second and
third degree coefficients, wherein a mean value is used as the third
degree coefficient and wherein each of the constant and the first and
second degree coefficients are determined from three temperature points
and frequency data measured at the three temperature points.
3. The method of claim 1, wherein the polynomial of degree n is
approximated by a cubic expression having a constant and first, second and
third degree coefficients, wherein a mean value is used as each of the
second and third degree coefficients, and wherein the first degree
coefficient and the constant are determined from two temperature points
and frequency data measured at the two temperature points.
4. In a digital control oscillation circuit having a CPU, a memory, a
temperature sensor, a crystal resonator, and a variable-capacitance diode,
wherein a change of a resonance frequency of the crystal resonator caused
by a temperature change is corrected by controlling a control voltage
applied to the variable-capacitance diode, a frequency correcting method
comprising:
providing a frequency deviation characteristic of the variable-capacitance
diode with respect to the control voltage in the form of a linear
expression for a linear portion which includes a point at which frequency
deviation is zero,
storing a first degree coefficient of the linear expression and a
temperature characteristic of the crystal resonator in the memory,
calculating, under control of the CPU, a frequency deviation from the
temperature characteristic of the crystal resonator of a temperature
detected by the temperature sensor,
obtaining a control voltage to be applied to the variable-capacitance diode
from the linear expression expressed by the first degree coefficient, and
applying the control voltage to the variable-capacitance diode,
whereby output frequency is controlled at a substantially constant
predetermined level.
5. In a digital control oscillation circuit including a CPU, a memory, a
temperature sensor, a crystal resonator, and a variable-capacitance diode,
and in which a change of a resonance frequency of the crystal resonator
caused by a temperature change is corrected by controlling a control
voltage applied to the variable-capacitance diode, thereby outputting a
constant frequency, a frequency correcting method comprising:
dividing a control voltage-frequency deviation characteristic of the
variable-capacitance diode into a plurality of appropriate control
sections,
expressing a frequency deviation in each control section as a linear
expression with respect to control voltage, and
storing a first degree coefficient of each linear expression, together with
a temperature characteristic of the crystal resonator, in the memory,
calculating, under control of the CPU, a frequency deviation from the
temperature characteristic of the crystal resonator according to a
temperature detected with the temperature sensor,
obtaining a control voltage to be applied to the variable-capacitance diode
by using the first degree coefficient for a corresponding control section,
and
applying the control voltage to the variable capacitance diode,
thereby controlling an output frequency at a substantially constant
predetermined level.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a digital control type oscillation circuit
of a portable telephone, and also relates to a crystal resonator
oscillation frequency calculating method and a frequency correcting
method, which are suitably used for the oscillation circuit.
FIG. 1 shows an example of the arrangement of a conventional digital
temperature-compensated crystal oscillator. As illustrated in the figure,
the digital temperature-compensated crystal oscillator 60 includes an
oscillation circuit 61 which uses a crystal resonator 45 (see FIG. 2), a
temperature sensor 62, an A/D converter 63, a CPU (Central Processing
Unit) 64, a D/A converter 65, an integrating circuit 66, and a storage
circuit (memory) 67.
Crystal resonators have the temperature characteristic that the oscillation
frequency changes with variations in temperature. Therefore, in the
oscillation circuit 61, which uses a crystal resonator as an oscillation
device, the oscillation frequency is corrected by applying a control
voltage corresponding to a temperature change, thereby maintaining the
output frequency at a constant level. For this purpose, the storage
circuit 67 has been previously stored with control information (voltage
values) for correcting a resonance frequency drift caused by a temperature
change. The temperature of the oscillation circuit 61 is detected with the
temperature sensor 62. The detected temperature is converted into a
digital value in the A/D converter 63 and then output to the CPU 64. The
CPU 64 refers to the correction information stored in the storage circuit
67 and outputs a control signal (digital signal) corresponding to the
detected temperature to the D/A converter 65. The D/A converter 65
converts the control signal into an analog value and outputs it to the
oscillation circuit 61 through the integrating circuit 66, thus
maintaining the output frequency at a predetermined constant level.
The digital temperature-compensated crystal oscillator 60 requires the
temperature sensor 62, the A/D converter 63, the CPU 64, the D/A converter
65, the integrating circuit 66 and the storage circuit 67, as described
above, and it is therefore complicated in arrangement and large in size.
Further, the digital temperature-compensated crystal oscillator 60 carries
out the temperature compensation in accordance with the temperature
characteristic of the crystal resonator 45. Therefore, in a case where the
digital temperature-compensated crystal oscillator 60 is used as an
oscillation circuit of a portable telephone, when the crystal resonator 45
has broken down, not the crystal resonator 45 alone but the whole
oscillator must be replaced with a new one; this is costly.
Further, when the digital temperature-compensated crystal oscillator 60 is
used as an oscillator of a portable telephone, the controlled object of
temperature compensation is the frequency at the output terminal of the
oscillation circuit 61. Therefore, the output frequency at the antenna of
the portable telephone undesirably has an error introduced in a
high-frequency section, e.g. a VCO.
FIG. 2 shows an example of the arrangement of the oscillation circuit 61,
shown in FIG. 1. The oscillation frequency of the oscillation circuit 61
is determined by the electrostatic capacities of the crystal resonator 45,
capacitors 43, 44, 46 and 47 and variable-capacitance diode 42. A
frequency signal oscillated from the crystal resonator 45 is amplified by
a transistor 50 and output from an output terminal T.sub.out through a
capacitor 56.
The crystal resonator 45 has the temperature characteristic that the
resonance frequency changes (on the order of .+-.10 ppm) with variations
in temperature. Therefore, the resonance frequency variation of the
crystal resonator 45 caused by temperature change is corrected by
controlling a voltage applied to the variable-capacitance diode 42 (i.e. a
voltage applied to an input terminal T.sub.in). It is necessary in order
to effect the frequency correction with high accuracy to calculate a
change in the resonance frequency of the crystal resonator 45 caused by a
temperature change with high accuracy. However, the frequency deviation of
an AT cut crystal resonator, for example, is expressed by a polynomial, as
described later. Therefore, data measured at a large number of measuring
points is needed for accurate calculation of a change in the resonance
frequency caused by a temperature change. Thus, a troublesome operation
must be carried out in order to prepare such measured data.
Further, in the conventional oscillation circuit, a resonance frequency
change of the crystal resonator caused by a temperature change is
corrected by controlling the voltage applied to the variable-capacitance
diode 42, as described above. However, each individual
variable-capacitance diode has its own control voltage-frequency deviation
characteristic; there are a variety of control voltage-frequency deviation
characteristics which differ in slope K, as shown in FIG. 3. The
conventional practice is to select a variable-capacitance diode having a
slope K close to a predetermined slope and to store the value thereof in
the memory 67 as the slope K of the variable-capacitance diode 42 in
advance. Accordingly, it takes a great deal of time to select a proper
variable-capacitance diode. Moreover, an error is introduced into the
control voltage (correction voltage) because the slope of the control
voltage-frequency deviation characteristic is not accurate. Further, the
slopes K of the straight lines representing control voltage-frequency
deviation characteristics are not uniform over the entire control voltage
range, as shown in FIG. 3. That is, the control voltage-frequency
deviation characteristic of each variable-capacitance diode is given by an
approximately linear expression. Accordingly, an error arises unless
correction is made using a control voltage which is given by a linear
expression with a slope K obtained in an appropriate control voltage
range.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a digital control type
oscillation circuit for use in a portable telephone, which is arranged
such that temperature compensation for a crystal oscillator is made by
effectively using control elements which have already been used in the
portable telephone, and that the controlled object of temperature
compensation is the output frequency at the antenna of the portable
telephone.
To attain the above-described object, the present invention provides a
digital control type oscillation circuit for use in a portable telephone
which uses a CPU, a memory, a temperature sensor, a D/A converter, and an
A/D converter as control elements. As shown in FIG. 4, the memory 20 is
previously stored with control information for correcting a drift of the
output frequency of the portable telephone caused by a temperature change.
The temperature of the oscillation circuit 22 is detected with the
temperature sensor 23. The detected temperature is converted into a
digital signal in the A/D converter 24, and the digital signal is input to
the CPU 1. The CPU 1 outputs a correction signal according to the control
information stored in the memory 20. The correction signal is converted
into an analog value in the D/A converter 21 and then input to the
oscillation circuit 22 as a control voltage, thereby controlling the
output frequency of the portable telephone at a predetermined constant
level. Thus, the temperature sensor 23, the A/D converter 24, the CPU 1,
the memory 20 and the D/A converter 21, which have already been provided
in the portable telephone, can also be used as control elements necessary
for temperature compensation. Further, the controlled object of
temperature compensation is the output frequency at the antenna 9, and it
can be used as temperature compensation data.
Another object of the present invention is to provide a method of
efficiently calculating an oscillation frequency of a crystal resonator
with a reduced number of points of measurement for a temperature
characteristic of the crystal resonator.
To attain the above-described object, the present invention provides a
method of calculating an oscillation frequency of a crystal resonator by
obtaining each coefficient of the temperature characteristic of the
crystal resonator, which is expressed in the form of a polynomial of
degree n with respect to temperature. The temperature characteristic,
which is expressed by a polynomial of degree n with respect to
temperature, is approximated with a cubic expression. A mean value is used
as the third-degree coefficient of the cubic expression, and the
second-degree coefficient, the first-degree coefficient and the constant
are determined from three temperature points and frequency data measured
at the three temperature points, thereby calculating an oscillation
frequency. Thus, by using a mean value as the third-degree coefficient, an
oscillation frequency deviation can be calculated with a sufficiently high
accuracy from data measured at three measuring points, whereas data
measured at four or more measuring points has heretofore been needed to
obtain the third-, second- and first-degree coefficients and the constant
and to calculate an oscillation frequency deviation.
The oscillation frequency calculating method may be carried out as follows:
A mean value is used as each of the third- and second-degree coefficients
of the cubic expression representing the temperature characteristic, and
the first-degree coefficient and the constant are determined from two
temperature points and frequency data measured at the two temperature
points, thereby calculating an oscillation frequency. Thus, by using a
mean value as each of the third- and second-degree coefficients, it is
possible to calculate an oscillation frequency deviation from data
measured at two measuring points. Accordingly, it is possible to reduce
the number of manhours and the cost on account of the reduction in the
number of data measuring points.
Still another object of the present invention is to provide a frequency
correcting method for a digital control type oscillation circuit, in which
a linear expression representing the relationship between the frequency
deviation and the control voltage is obtained for each
variable-capacitance diode to be used for temperature compensation in
order to effect voltage control, thereby improving the frequency accuracy.
To attain the above-described object, as shown in FIG. 10, a frequency
deviation characteristic relative to the control voltage is expressed in
the form of a linear expression for a linear portion including a point at
which the frequency deviation is zero for each variable-capacitance diode
to be used, and the first-degree coefficient of the linear expression and
the temperature characteristic of a crystal resonator are stored in a
memory in advance. A CPU calculates a frequency deviation from the
temperature characteristic of the crystal resonator according to the
temperature detected with a temperature sensor, obtains a control voltage
to be applied to the variable-capacitance diode from the linear expression
expressed by the first-degree coefficient, and applies the control voltage
to the variable-capacitance diode, thereby controlling the output
frequency at a predetermined constant level. Thus, according to the method
of the present invention, the frequency deviation characteristic relative
to the control voltage is expressed in the form of a linear expression for
a linear portion including a point at which the frequency deviation is
zero for each variable-capacitance diode to be used, and a slope K is set
according to measured values. Therefore, accurate frequency control can be
effected without an error which has heretofore been caused by
characteristic variation.
A further object of the present invention is to provide a frequency control
method for a digital control type oscillation circuit which makes it
possible to readily use even a crystal resonator having a
frequency-temperature characteristic which differs from a reference
characteristic to a considerable extent.
To attain the above-described object, the present invention provides a
method of controlling the oscillation frequency of a digital control type
oscillation circuit having a control section which is adapted to correct a
drift of the resonance frequency of a crystal resonator, which is caused
by a temperature change, by detecting a temperature change of the crystal
resonator with a temperature sensor and controlling a voltage to be
applied to a variable-capacitance element, thereby controlling the
oscillation frequency of the oscillation circuit at a constant level. The
control section is provided with a storage circuit. At least three
appropriate temperature points are selected, and control voltages for
correcting a frequency deviation of the crystal resonator are measured at
the selected temperature points. Then, a cubic correction curve is
determined from the values measured at the measuring points, and the
coefficients of the correction curve are stored in the storage circuit in
advance. A control voltage corresponding to a temperature measured with
the temperature sensor is calculated from the cubic correction curve, and
the control voltage is applied to the variable-capacitance element in
order to control the electrostatic capacity of the variable-capacitance
element, thereby maintaining the oscillation frequency of the oscillation
circuit at a constant level.
The frequency control method may be carried out as follow: Three
appropriate temperature points are selected, and a control voltage for
correcting a frequency deviation of the crystal resonator is measured at
each of the three temperature points. Then, a correction curve represented
by a cubic expression is determined from the values measured at the three
temperature points, and the coefficients of the cubic correction curve are
stored in the storage circuit in advance. A control voltage corresponding
to a temperature measured with a temperature sensor is calculated from the
cubic correction curve, and the control voltage is applied to the
variable-capacitance element in order to control the electrostatic
capacity of the variable-capacitance element, thereby maintaining the
oscillation frequency of the oscillation circuit at a constant level.
According to the above-described method, a frequency correction curve is
calculated from values measured at three measuring points, and the
coefficients (temperature coefficients) of the frequency correction curve
are stored in the storage circuit. Therefore, the required storage
capacity is minimized, and the number of manhours is also reduced.
Accordingly, it is possible to construct an oscillation circuit capable of
accurate temperature compensation at reduced cost. Further, it is possible
to readily use even a crystal resonator having a frequency-temperature
characteristic which differs from a reference characteristic to a
considerable extent because a correction curve for each crystal resonator
is directly obtained. Thus, the frequency control method is superior in
flexibility to the conventional technique.
A still further object of the present invention is to provide a frequency
correcting method for a digital control type oscillation circuit which
achieves an improvement in the frequency accuracy by controlling a control
voltage to be applied to a variable-capacitance diode used for temperature
compensation in such a manner that the control voltage-frequency deviation
characteristic of the variable-capacitance diode is divided into a
plurality of control sections, and the control voltage is controlled on
the basis of data stored for a particular control section.
To attain the above-described object, as shown in FIG. 19, the control
voltage-frequency deviation characteristic of a variable-capacitance diode
to be used for temperature compensation is appropriately divided into a
plurality of control sections (three sections in the illustrated example).
The frequency deviation in each control section is given by a linear
expression with respect to control voltage, and the first-degree
coefficient of the linear expression and the temperature characteristic of
the crystal resonator are stored in a memory in advance. A CPU calculates
a frequency deviation from the temperature characteristic of the crystal
resonator according to a temperature detected with a temperature sensor,
obtains a control voltage to be applied to the variable-capacitance diode
by using the first-degree coefficient in the corresponding control
section, and applies the control voltage to the variable-capacitance
diode, thereby controlling the output frequency at a predetermined
constant level.
Thus, according to the above-described method, the control
voltage-frequency deviation characteristic of a variable-capacitance diode
to be used for temperature compensation is divided into a plurality of
control sections, and a slope K of the variable-capacitance diode is set
for each control section on the basis of measured values. Therefore,
frequency deviations (errors) reduce in all the control sections, and thus
it is possible to effect accurate frequency control.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the arrangement of a conventional digital
temperature-compensated crystal oscillator.
FIG. 2 shows the arrangement of an oscillation circuit which uses a crystal
resonator.
FIG. 3 shows control voltage-frequency deviation characteristics of various
variable-capacitance diodes.
FIG. 4 shows the arrangement of a portable telephone which uses a digital
control type oscillation circuit according to the present invention.
FIG. 5 shows the structure of an AT cut crystal plate.
FIG. 6 shows the frequency deviation-temperature characteristics of AT cut
crystal resonators with which the present invention is concerned.
FIG. 7 shows a characteristic curve obtained from data measured at three
measuring points with coefficient A.sub.3 fixed, together with measured
values.
FIG. 8 shows calculated values and measured values, which were obtained
when coefficient A.sub.3 was changed by -8%.
FIG. 9 shows temperature characteristics of crystal resonators.
FIG. 10 shows the characteristic of a variable-capacitance diode used in a
frequency correcting method according to the present invention.
FIG. 11 shows the control voltage relative to the temperature obtained by a
frequency correcting method according to the present invention.
FIG. 12 shows the control voltage relative to the temperature obtained by a
frequency correcting method according to the present invention.
FIG. 13 shows the temperature characteristic of a crystal resonator.
FIG. 14 shows a correction curve for correcting a frequency deviation
caused by a temperature change.
FIG. 15 shows an example in which estimated values are obtained values
measured at three measuring points, and a cubic correction curve is
obtained from these values.
FIG. 16 shows frequency-temperature characteristics before and after
frequency correction made by a control method according to the present
invention.
FIG. 17 shows the control voltage relative to the temperature obtained by a
conventional frequency correcting method.
FIG. 18 shows the control voltage relative to the temperature obtained by a
conventional frequency correcting method.
FIG. 19 shows the characteristic of a variable-capacitance diode used in a
frequency correcting method according to the present invention.
FIG. 20 shows the control voltage relative to the temperature obtained by a
frequency correcting method according to the present invention.
FIG. 21 shows the control voltage relative to the temperature obtained by a
frequency correcting method according to the present invention.
FIG. 22 shows temperature characteristics obtained by a conventional
frequency correcting method.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
One embodiment of the present invention will be described below in detail
with reference to the accompanying drawings. FIG. 4 is a block diagram
showing an example of the arrangement of a portable telephone which uses a
digital control type oscillation circuit according to the present
invention. As illustrated in the figure, a portable telephone which uses a
digital control type oscillation circuit according to the present
invention includes a CPU (Central Processing Unit) 1, a data signal
processing unit 2, a baseband section 3 for transmitting and receiving
processing, a modulator 4, a mixer 5 serving as a frequency converter, an
amplifier 6, an output control section 7, an antenna multiplexer 8, an
antenna 9, an amplifier 10, a mixer 11, an amplifier 12, a mixer 13, a
demodulator 14, a PPL (Phase Locked Loop) 15, a VCO (Voltage-Controlled
Oscillator) 16, a PPL 17, a VCO 18, an automatic output controller 19, a
memory 20 for storing data, a D/A converter 21, an oscillation circuit 22,
a temperature sensor 23, an A/D converter 24, a display unit 30, a keypad
31 for inputting a dial signal, an interface 32 for connection with an
external device, a telephone transmitter 33, and a telephone receiver 34.
It should be noted that the oscillation circuit 22 is the same as that
shown in FIG. 2.
A dial signal input from the keypad 31 is processed in the CPU 1 and
displayed on the display unit 30. The signal that is processed in the CPU
1 is further processed in the data signal processing unit 2 and the
baseband section 3 as a signal to be transmitted. Then, the signal is
modulated in the modulator 4, frequency-converted in the mixer 5,
amplified in the amplifier 6, and supplied to the antenna 9 via the output
control section 7 and the antenna multiplexer 8. Then, the signal is
transmitted from the antenna 9. A speech signal to be transmitted is
supplied from the telephone transmitter 33 to the baseband section 3, and
it is similarly processed and transmitted.
A signal from the other party is received by the antenna 9 and supplied to
the amplifier 10 via the antenna multiplexer 8. The signal amplified in
the amplifier 10 is frequency-converted in the mixer 11, amplified in the
amplifier 12, frequency-converted in the mixer 13, demodulated in the
demodulator 14, processed in the baseband section 3 as a received signal,
and output from the telephone receiver 34 as speech.
The oscillation circuit 22 outputs a constant frequency, and the output
frequency is supplied to the modulator 4 and the PPL 15. Thus, the output
frequency of the VCO 16 is controlled by the output of the PPL 15. The
mixer 5 mixes together the output signal of the modulator 4 and the output
frequency of the VCO 16, thereby converting the frequency of the output
signal to a predetermined transmit frequency. The CPU 1 controls the PPL
15 so as to oscillate a predetermined frequency according to the working
channel. The received signal from the amplifier 10 is also mixed with the
output frequency of the VCO 16 in the mixer 11, thereby being
frequency-converted.
The oscillation circuit 22 also outputs a constant frequency to the PPL 17
to control the output frequency of the VCO 18. The mixer 13 mixes together
the received signal from the amplifier 12 and the output frequency of the
VCO 18, thereby converting the frequency of the received signal to a
predetermined frequency. The CPU 1 adjusts the PPL 17 to thereby control
the VCO 18 so that the frequency of the received signal is converted to a
predetermined frequency according to the receive channel frequency.
Next, the temperature compensating control of the oscillation circuit 22
will be explained below. The memory 20 has been previously stored with
correction data (control voltage values) for correcting a drift of the
output frequency at the antenna 9 caused by a temperature change. The
temperature sensor 23 detects a temperature of the oscillation circuit 22.
The detected temperature is converted into a digital value in the A/D
converter 24 and then output to the CPU 1. The CPU 1 refers to the
correction data stored in the memory 20, and outputs a control voltage
value (digital signal) corresponding to the detected temperature to the
D/A converter 21. The D/A converter 21 converts the control voltage, which
is a digital signal, into an analog value, and outputs it to the
oscillation circuit 22 to control the frequency, thereby maintaining the
output frequency at the antenna 9 at a predetermined constant level.
Portable telephones are usually used under severe environmental conditions.
Therefore, the temperature sensor 23 for detecting a temperature has
heretofore been used so that the portable telephone is adaptable to
changes in temperature. The D/A converter 21 has also heretofore been used
as a device for converting a signal to be input to the automatic output
controller 19 or a digital speech signal into an analog signal. The A/D
converter 24 has also been used as a device for converting speech into a
digital signal. Needless to say, the CPU 1 and the memory 20 have also
been used for control.
The portable telephone arranged as shown in FIG. 4 is provided with the
same temperature compensating function as that of the digital
temperature-compensated crystal oscillator arranged as shown in FIG. 1 by
making use of the temperature sensor 23, the D/A converter 21, the A/D
converter 24, the CPU 1 and the memory 20, which have already been
provided in the portable telephone as control elements. That is, these
control elements, which have already been provided in the portable
telephone, are used for temperature compensation of the oscillation
circuit 22 in common with the portable telephone, thereby achieving
reduction in size and cost of the portable telephone.
In a portable telephone which incorporates the conventional digital
temperature-compensated crystal oscillator arranged as shown in FIG. 1,
the controlled object is the output frequency of the oscillation circuit,
whereas, in the portable telephone shown in FIG. 4, the controlled object
is the output frequency at the antenna 9. Therefore, it is possible to
realize even more accurate control than in the conventional system.
Further, in the above-described conventional portable telephone,
temperature compensation is made in accordance with the temperature
characteristic of a crystal resonator used therein. Therefore, when the
crystal resonator, i.e. the oscillation circuit 22, has broken down, not
the oscillation circuit 22 alone but the whole digital
temperature-compensated crystal oscillator, arranged as shown in FIG. 1,
must be replaced, whereas, in the portable telephone shown in FIG. 4, only
the broken oscillation circuit 22 is necessary to replace.
As has been described above, a crystal resonator is used in the oscillation
circuit of a portable telephone. A crystal plate cut for use as a crystal
resonator also has the temperature characteristic that the oscillation
frequency changes with variations in temperature. The temperature
characteristics of crystal plates differ according to the manner in which
the crystal plates are cut, i.e. AT cut, SC cut, and CT cut. FIG. 5 shows
the structure of an AT cut crystal plate. As illustrated in the figure,
the crystal resonator has electrodes a provided on both sides thereof. The
natural frequency of the crystal resonator is determined by the material
and configuration c, the cutting angle b, and the thickness d.
FIG. 6 shows frequency deviation-temperature characteristics of AT cut
crystal resonators. As illustrated in the figure, the crystal resonator
having the characteristic B exhibits a very good characteristic in the
temperature range of from -10.degree. C. to +60.degree. C., whereas the
crystal resonator having the characteristic A shows smaller frequency
variations in the temperature range of from -50.degree. C. to +100.degree.
C. than the other two crystal resonators. Accordingly, a crystal resonator
having an appropriate characteristic is used according to the use
application and the working temperature range.
As shown in FIG. 6, the frequency deviation .DELTA.f/f (ppm) of an AT cut
crystal resonator is generally given by the following polynomial:
.DELTA.f/f=A.sub.0 +A.sub.1 T+A.sub.2 T.sup.2 +A.sub.3 T.sup.3 +A.sub.4
T.sup.4 +A.sub.5 T.sup.5 + (1)
where .DELTA.f is a frequency deviation, f is an oscillation frequency,
A.sub.0 to A.sub.n are coefficients, and T is a temperature.
In actual calculation of the above expression (1), the terms higher than
third order are ignored because they are sufficiently small, and a
frequency deviation can be calculated with sufficiently high accuracy by
the following cubic expression with respect to temperature:
.DELTA.f/f=A.sub.0 +A.sub.1 T+A.sub.2 T.sup.2 +A.sub.3 T.sup.3(2)
Accordingly, for the frequency control, the constant A.sub.0 and
coefficients A.sub.1, A.sub.2 and A.sub.3 of the crystal resonator are
previously obtained. During the oscillation operation, the temperature of
the crystal resonator is detected, and the expression (2) is calculated to
obtain a frequency deviation .DELTA.f/f. Then, the frequency deviation is
corrected by the crystal oscillation circuit shown in FIG. 2. By doing so,
a constant frequency can be maintained.
The crystal resonator 45 has the temperature characteristic that the
resonance frequency changes (on the order of .+-.10 ppm) with variations
in temperature, as has been described above. The variable-capacitance
diode 42 is a device whose electrostatic capacity changes with the control
voltage applied thereto. A characteristic curve showing the relationship
between the control voltage applied to the variable-capacitance diode 42
and the change of the output frequency is previously obtained by
measurement. When the temperature of the crystal resonator 45 has changed
during the oscillating operation of the oscillation circuit, a control
unit (not shown) calculates a frequency deviation by the expression (2),
and controls the voltage applied to the input terminal (see FIG. 2) so as
to correct the frequency deviation, thereby controlling the output
frequency at a constant level.
The conventional oscillation frequency control needs to previously obtain
and store the constant A.sub.0 and coefficients A.sub.1, A.sub.2 and
A.sub.3 of the crystal resonator and the characteristic of the
variable-capacitance diode 42. Accordingly, it has heretofore been
necessary to obtain the coefficients A.sub.0, A.sub.1, A.sub.2 and A.sub.3
of each crystal resonator from data measured at four or more measuring
points in advance. It should be noted that the constant A.sub.0 in the
expression (2) is practically determined by the thickness d of the crystal
plate (see FIG. 3), and that the coefficient A.sub.1 is determined by the
cutting angle b, and further that the coefficients A.sub.2 and A.sub.3 are
practically determined by the material and configuration c.
However, the above-described AT cut crystal resonator differs in
temperature characteristic according to the specifications; the constant
A.sub.0 and coefficients A.sub.1, A.sub.2 and A.sub.3 differ for each
crystal resonator. Further, it is necessary in order to obtain four values
for the constant A.sub.0 and coefficients A.sub.1, A.sub.2 and A.sub.3 to
prepare data measured at four or more measuring points for each crystal
resonator. Preparation of such measured data requires a complicated
operation and causes an increase in the number of manhours and a rise in
cost. Moreover, the working efficiency is inferior, and the incidence of
errors increases.
Accordingly, in this embodiment an efficient crystal resonator oscillation
frequency calculating method is provided in which the number of measuring
points required to measure a temperature characteristic of a crystal
resonator is reduced.
The crystal resonator oscillation frequency calculating method will be
explained below in detail. Variations in values for the constant A.sub.0
and coefficients A.sub.1, A.sub.2 and A.sub.3 of AT cut crystal resonators
made of the same material and produced by the same production process fall
within a relatively narrow range. For example, variations in values for
the coefficient A.sub.3 fall within .+-.8%. The variations may be said to
be manufacturing errors introduced during the production of AT cut crystal
resonators.
FIG. 7 shows a characteristic curve obtained from data measured at three
measuring points by the oscillation frequency calculating method according
to the present invention by using a mean value as the coefficient A.sub.3.
FIG. 7 also shows measured values. The characteristic curve shown in FIG.
7 was obtained as follows: With the coefficient A.sub.3 fixed at a mean
value (A.sub.3 =-0.15/10.sup.5) obtained by calculation, frequencies were
measured at three temperature points of -20.degree. C., 20.degree. C. and
60.degree. C., respectively, and the constant A.sub.3 and coefficients
A.sub.1 and A.sub.2 were obtained from the following expression (3),
thereby calculating a characteristic curve:
Y.sub.1 =A.sub.0 +A.sub.1 T.sub.1 +A.sub.2 T.sub.1.sup.2 +A.sub.3
T.sub.1.sup.3
Y.sub.2 =A.sub.0 +A.sub.1 T.sub.2 +A.sub.2 T.sup.2.sup.2 +A.sub.3
T.sub.2.sup.3 (3)
Y.sub.3 =A.sub.0 +A.sub.1 T.sub.3 +A.sub.2 T.sub.3.sup.2 +A.sub.3
T.sub.3.sup.3
where
T.sub.1 : a measuring temperature of -20.degree. C.
T.sub.2 : a measuring temperature of 20.degree. C.
T.sub.3 : a measuring temperature of 60.degree. C.
Y.sub.1 : a frequency deviation measured at -20.degree. C.
Y.sub.2 : a frequency deviation measured at 20.degree. C.
Y.sub.3 : a frequency deviation measured at 60.degree. C.
As will be understood from FIG. 7, if an appropriate value is selected as
the coefficient A.sub.3, a frequency deviation can be calculated to be
accurate to within 0.1 ppm.
The measuring temperatures are not necessarily limited to the three points
of -20.degree. C., 20.degree. C. and 60.degree. C. If appropriate values
are selected in the range of from -20.degree. C. to 75.degree. C., a
frequency deviation can be calculated to be accurate to within 0.1 ppm
with respect to the measured value (not shown) in the same way as the
above.
The cubic curve representing the temperature characteristic of the AT cut
crystal resonator is centered at approximately 25.degree. C. (point of
inflection). The reason for this is to make the characteristic at and near
room temperatures as smooth as possible. Consequently, frequency
variations are relatively large at high and low temperatures. Therefore,
it is preferable to select measuring temperatures at higher and lower
levels in the working temperature range (of portable telephones). By doing
so, frequency variations become uniform as a whole, and thus favorable
results are obtained.
Crystal resonators have the same value for the coefficient A.sub.3 if there
is no manufacturing error. However, the result of measuring a large number
of existing AT cut crystal resonators reveals that there are manufacturing
errors of .+-.8% even if the specifications are the same. FIG. 8 shows
calculated values and measured values, which were obtained when the
coefficient A.sub.3 was changed from the mean value by -8%.
The calculated values in FIG. 8 were obtained by determining the
coefficients A.sub.0, A.sub.1 and A.sub.2 from the expression (3) and thus
calculating a frequency deviation characteristic curve. As will be
understood from FIG. 8, a frequency deviation can be calculated with an
error of 0.2 ppm in the measuring temperature range of from -20.degree. C.
to 60.degree. C. With respect to temperatures outside that temperature
range, a frequency deviation can be calculated with an error of 0.35 ppm
at the point of -30.degree. C., with an error of 0.35 ppm at the point of
70.degree. C., and with an error of about 0.6 ppm at the point of
75.degree. C.
In another experiment, with the value of the coefficient A.sub.3 increased
by .+-.8% from the mean value, frequencies were measured at three
temperature points which were 20.degree. C. apart in the range of from
-20.degree. C. to 75.degree. C., and the coefficients A.sub.0, A.sub.1 and
A.sub.2 were obtained to calculate frequency deviations. In this case
also, results similar to those described above were obtained. These
results satisfactorily meet the requirements that the frequency deviation
shall be within .+-.1 ppm in the temperature range of from -20.degree. C.
to 60.degree. C., as specified by RCR-STD (RCR Standards).
As has been described above, by using a mean value as a value for the
coefficient A.sub.3 of a crystal resonator, a frequency deviation can be
readily calculated by obtaining the other coefficients A.sub.0, A.sub.1,
and A.sub.2 from data measured at three measuring points. Although in the
foregoing description a mean value is used as a value for the coefficient
A.sub.3 of a crystal resonator, it should be noted that, it is also
possible to use a mean value as a value for the coefficient A.sub.2 and to
obtain the other coefficients A.sub.0, A.sub.1 and A.sub.3 from data
measured at three measuring points. Alternatively, it is also possible to
use a mean value as a value for each of the coefficients A.sub.2 and
A.sub.3 of a crystal resonator and to obtain the constant A.sub.0 and the
coefficient A.sub.1 from data measured at two measuring points.
Next, a specific example of calculating a frequency deviation will be
explained. A frequency deviation is calculated in order to obtain a
control voltage (correction voltage) to be applied to the input terminal
T.sub.in of the crystal oscillation circuit shown in FIG. 2; generally, it
is calculated by a processing unit (CPU). When the temperature of the
crystal resonator 45 has changed during the oscillating operation of the
crystal oscillation circuit, the processing unit (CPU) receives
information from the temperature sensor, and calculates a frequency
deviation by the cubic expression (A.sub.0 +A.sub.1 T+A.sub.2 T.sup.2
+A.sub.3 T.sup.3) shown by Eq. (2). In order to reduce the computing time
required for the calculation, each coefficient is multiplied by 10.sub.n,
and the resulting product is rounded off to the first decimal place,
thereby converting the coefficient into a value of integer type. This
integer coefficient is stored in the memory in advance. When a frequency
deviation is to be calculated, the processing unit (CPU) divides the
integer coefficient by 10.sub.n, thereby preventing an error from being
introduced into the result of the calculation. By doing so, the computing
time can be reduced to 1/15 of that in the case of floating-point
calculation, and thus the processing time can be reduced to a considerable
extent.
In this embodiment, the number of items of measured data can be reduced by
using mean values for some of n coefficients. For example, if a mean value
is used as the third-degree coefficient, a frequency deviation value can
be calculated with sufficiently high accuracy by obtaining each
coefficient from data measured at three measuring points, although data
measured at four measuring points has heretofore been needed to obtain the
third-, second- and first-degree coefficients and the constant and to
thereby calculate an oscillation frequency deviation value.
If a mean value is used as each of the third- and second-degree
coefficients, a frequency deviation value can be calculated with
sufficiently high accuracy by obtaining each coefficient from data
measured at two measuring points. Accordingly, it is possible to reduce
the number of manhours and the cost as a result of the reduction in the
number of points for measuring data.
Next, a method of correcting the frequency of a digital control type
oscillation circuit of a portable telephone according to the present
invention will be explained.
FIG. 9 shows temperature characteristics of crystal resonators. As will be
understood from the figure, crystal resonators vary from each other in
temperature characteristics. FIG. 9 shows the temperature characteristics
of crystal resonators whose resonance frequencies change in the range of
from -12 ppm to +7 ppm at the maximum relative to a reference frequency
when the temperature is changed in the range of from -30.degree. C. to
75.degree. C. with 25.degree. C. defined as a center. In order to correct
the frequency deviation of a crystal resonator, the control voltage
applied to the variable-capacitance diode 42 is adjusted by the
oscillation circuit in FIG. 2.
The control voltage (V.sub.cont)-frequency deviation characteristic of a
variable-capacitance diode is expressed by an approximately linear
expression with respect to control voltage as shown in FIG. 3. The slope
of the control voltage-frequency deviation characteristic of the
variable-capacitance diode 42 is assumed to be K.
Accordingly, a frequency deviation can be corrected by applying a control
voltage (correction voltage) V.sub.i given by
V.sub.i =V.sub.0 -.DELTA.f.sub.i .multidot.K (4)
where V.sub.0 is a control voltage when the frequency deviation is zero,
.DELTA.f.sub.i is a frequency deviation at each temperature, V.sub.i is a
control voltage (correction voltage) to be applied at each particular
temperature, and K is the slope of the variable-capacitance diode 42.
The conventional frequency correcting method has heretofore been carried
out as follows: As shown in FIGS. 1 and 2, the memory 67 has been
previously stored with the temperature characteristic (the relation
between the temperature change and the frequency deviation .DELTA.f.sub.i,
or a reference table concerning the temperature characteristic) of the
crystal resonator 45, together with the value of the slope K of the
variable-capacitance diode 42. The CPU 64 calculates a control voltage
(correction voltage) V.sub.i by the linear expression (4) according to the
temperature detected with the temperature sensor 62, and applies the
calculated control voltage V.sub.i to the variable-capacitance diode 42,
thereby correcting a drift of the output frequency.
However, the slope K of the variable-capacitance diode 42 varies from that
of another, as shown in FIG. 3. The conventional practice is to select a
variable-capacitance diode having a slope close to a predetermined slope K
and to store the value thereof in the memory 67 as the slope K of the
variable-capacitance diode 42 in advance. Accordingly, the conventional
practice has the problem that it is not possible to use any
variable-capacitance diode but one which has the slope K. Moreover, an
error is introduced into the control voltage (correction voltage) V.sub.i
because the slope of the control voltage-frequency deviation
characteristic is not accurate. As shown for example in FIG. 22,
variations in the slope K cause the control voltage to have a maximum
error of about 0.15 V, i.e. a maximum error of about 5 ppm in terms of the
frequency deviation.
Further, the slopes K of the straight lines representing control
voltage-frequency deviation characteristics of variable-capacitance diodes
as shown in FIG. 3 are not uniform over the entire control voltage range;
the control voltage-frequency deviation characteristic of each
variable-capacitance diode is given by an approximately linear expression.
Accordingly, an error arises unless correction is made using a control
voltage (correction voltage) V.sub.i which is given by a linear expression
with a slope K obtained in an appropriate control voltage range. These
errors, considered together with other error factors of the crystal
resonator 45, exceed 1 ppm and do not meet the requirements that the
frequency deviation shall be within .+-.1 ppm in the temperature range of
from -20.degree. C. to 60.degree. C., as specified by RCR-STD (RCR
Standards).
Therefore, in the present invention, a linear expression representing the
relationship between the frequency deviation and the control voltage is
obtained for each variable-capacitance diode to be used for temperature
compensation, and the control voltage is controlled according to the
linear expression, thereby achieving an improvement in the accuracy of the
frequency. The arrangement of a digital temperature-compensated crystal
oscillator, to which the frequency correcting method according to the
present invention is applied, and the oscillation circuit are the same as
those shown in FIGS. 1 and 2; therefore, description thereof is omitted.
FIG. 10 shows the characteristic of a variable-capacitance diode used in
the frequency correcting method according to the present invention. As
shown by the graph of the measured values a in the figure, the
characteristic of the variable-capacitance diode 42 becomes nonlinear as
the distance from the center increases. In this embodiment, however, the
slope K of the variable-capacitance diode 42 is approximated with a
straight line b including the point at which the frequency deviation is
zero. The slope K is given by
K=(V-V.sub.0)/.DELTA.f (5)
where V.sub.0 is a control voltage when the frequency deviation is zero, V
is a control voltage, and .DELTA.f is a frequency deviation at the control
voltage V.
A control voltage (correction voltage) to be applied to the
variable-capacitance diode 42 is calculated by the above expression (4)
(V.sub.i =V.sub.0 -.DELTA.f.sub.i .multidot.K), and the calculated control
voltage is applied to the variable-capacitance diode 42.
The crystal resonator 45 exhibits a temperature characteristic such as that
shown in FIG. 9, which is generally given by a cubic expression with
respect to temperature. The slope K of the variable-capacitance diode 42
and the temperature characteristic (cubic expression) of the crystal
resonator 45 are previously stored in the memory 67. The CPU 64 calculates
a frequency deviation from the temperature detected with the temperature
sensor 62 and the temperature characteristic of the crystal resonator 45.
Then, the CPU 64 calculates a control voltage by the expression (4) using
the slope K of the variable-capacitance diode 42, and applies the
calculated control voltage to the variable-capacitance diode 42.
FIG. 11 shows the way in which the control voltage (V.sub.cont) is changed
relative to the temperature by the frequency correcting method according
to the present invention. In the case of using a variable-capacitance
diode having an approximately linear control voltage-frequency deviation
characteristic as shown in FIG. 3 and a crystal resonator having a
temperature characteristic such as that shown by MIN in FIG. 9, the
frequency deviation (error) can be kept within 0.6 ppm by changing the
control voltage relative to the temperature as shown in FIG. 11.
FIG. 12 also shows the way in which the control voltage (V.sub.cont) is
changed relative to the temperature by the frequency correcting method
according to the present invention. In the case of using a
variable-capacitance diode having an approximately linear control
voltage-frequency deviation characteristic as shown in FIG. 3 and a
crystal resonator having a temperature characteristic such as that shown
by MAX in FIG. 9, the frequency deviation (error) can also be kept within
0.5 ppm by changing the control voltage relative to the temperature as
shown in FIGS. 11 and 12.
Thus, according to the method of the present invention, a frequency
deviation characteristic with respect to the control voltage is expressed
by a linear expression for a linear portion including a point at which the
frequency deviation is zero for each variable-capacitance diode 42 to be
used, and a slope K is set on the basis of measured values. Therefore,
there is no error due to variations as in the conventional practice, and
it becomes possible to effect accurate frequency control.
Further, because the control voltage-frequency deviation characteristic is
represented by a signal straight line over the entire control range, the
measurement is facilitated, and there is no need for a selecting operation
which has heretofore been needed. It should be noted that an optimum
voltage range for favorably obtaining the slope K is from 1.8 V to 3.5 V.
Further, according to the method of the present invention, frequency
correction can be realized simply by storing the necessary coefficients in
the memory using the conventional hardware as it is. Therefore, no extra
cost is required. Furthermore, because no feedback control is used, the
control operation is rapidly stabilized. Therefore, the present invention
is most suitable for use as a frequency correcting method for portable
telephones.
Next, another method of correcting the frequency of a digital control type
oscillation circuit of a portable telephone according to the present
invention will be explained.
As described above, crystal resonators have the temperature characteristic
that the resonance frequency changes (on the order of .+-.10 ppm) with
variations in temperature, and the variable-capacitance diode 42 is a
device whose electrostatic capacity changes with the control voltage
applied thereto. Accordingly, when the temperature of the crystal
resonator 45 has changed, the CPU 64 (see FIG. 1) controls the
electrostatic capacity of the variable-capacitance diode 42 by controlling
the voltage applied to the variable-capacitance diode 42 of the
oscillation circuit 61 (see FIG. 2). By doing so, it is possible to
control the output frequency (oscillation frequency) at a constant level.
FIG. 13 shows an example of the resonance frequency-temperature
characteristic of a crystal resonator. The axis of ordinate represents the
deviation of the resonance frequency from a reference frequency.
Generally, the resonance frequency-temperature characteristic can be
approximately represented by a cubic expression. FIG. 14 shows a
correction curve for correcting a frequency deviation caused by a
temperature change. The axis of abscissa represents the temperature, and
the axis of ordinate represents the deviation of the control voltage,
which is applied to the variable-capacitance diode 42, from a reference
control voltage. The graph of FIG. 14 shows that a control voltage
deviation corresponding to a temperature is input to the input terminal
T.sub.in (see FIG. 2) so as to correct the output frequency at the output
terminal T.sub.out, thereby maintaining the output frequency at a constant
level.
Crystal resonators which may be used as the crystal resonator 45 in FIG. 2
differ from each other in temperature characteristics. Therefore, the
frequency deviation must be corrected for each crystal resonator. FIG. 15
shows an example in which a cubic correction curve is determined by
obtaining estimated values from values measured at three points. The
frequency correcting method will be explained below with reference to FIG.
15.
(1) Input terminal voltage (control voltage applied to the
variable-capacitance diode 42) at which a predetermined output frequency
is obtained is measured at each of measuring temperature points of
-20.degree. C., room temperature (10.degree. C. to 35.degree. C.) and
60.degree. C. The measured input terminal voltages are assumed to be
V.sub.-20, V.sub.NT, and V.sub.60, respectively. Subsequently, the steps
(2) to (4) are carried out. For these steps (2) to (4), three different
methods are available, which will be shown in the following paragraphs A,
B and C.
A. (2) The differences V.sub.-20-NT and V.sub.60-NT are determined:
V.sub.-20-NT =V.sub.-20 -V.sub.NT 1
V.sub.60-NT =V.sub.60 -V.sub.NT 2
(3) Values for V.sub.-30-NT and V.sub.75-NT are estimated according to a
linear multiple regression model by using the differences V.sub.-20-NT and
V.sub.60-NT obtained from Eqs. 1 and 2 as variables.
(4) V.sub.NT is added to the values for V.sub.-30-NT and V.sub.75-NT
estimated at (3) to obtain values for V.sub.-30 and V.sub.75 (see the
estimated values shown in FIG. 15).
V.sub.-30 =V.sub.-30-NT +V.sub.NT 3
V.sub.75 V.sub.75-NT +V.sub.NT 4
B. (2) The ratios V.sub.-20/NT and V.sub.60/NT are determined:
V.sub.-20/NT =V.sub.-20 /V.sub.NT 1
V.sub.60/NT =V.sub.60 /V.sub.NT 2
(3) Values for V.sub.-30/NT and V.sub.75/NT are estimated according to a
linear multiple regression model by using the ratios V.sub.-20/NT and
V.sub.60/NT obtained from Eqs. 1 and 2 as variables.
(4) The values for V.sub.-30/NT and V.sub.75/NT estimated at (3) are
multiplied by V.sub.NT to obtain values for V.sub.-30 and V.sub.75 (see
the estimated values shown in FIG. 15).
V.sub.-30 =V.sub.-30/NT .times.V.sub.NT 3
V.sub.75 =V.sub.75/NT .times.V.sub.NT 4
C. (2) Values for V.sub.-30/NT and V.sub.75/NT are determined according to
a linear multiple regression model by using V.sub.-20, V.sub.NT and
V.sub.60 as variables (see the estimated values shown in FIG. 15).
There is no step (3) nor (4).
(5) A correction curve which is represented by a cubic expression is
prepared by using the method of least squares from the measured input
terminal voltages (control voltages to be applied to the
variable-capacitance diode 42) V.sub.-20, V.sub.NT and V.sub.60 and the
input terminal voltages V.sub.-30 and V.sub.75 obtained by the estimation
using a linear multiple regression model. It should be noted that the
ordinate axis of the graph shown in FIG. 15 represents the control voltage
deviation. It should also be noted that, when values measured at four or
more measuring points are used, the above-described step for obtaining
estimated values is omitted.
Each coefficient (temperature coefficient) of the cubic correction curve
obtained by the above-described method is previously stored in the storage
circuit 67 shown in FIG. 1. The CPU 64 calculates the cubic expression on
the basis of the temperature detected with the temperature sensor 62 and
input thereto through the A/D converter 63 to obtain a control voltage
deviation, and delivers the calculated control voltage deviation as an
output signal. The output signal is converted into an analog value in the
D/A converter 65, integrated in the integrating circuit 66, and input to
the oscillation circuit 61. The input voltage is applied to the
variable-capacitance diode 42 in the oscillation circuit 61 to control the
output frequency (see FIG. 2).
FIG. 16 shows frequency-temperature characteristics before and after
correction made by a control method according to the present invention. In
the figure, the curve A (continuous line) represents the
frequency-temperature characteristic after the correction, and the curve B
(dashed line) represents the frequency-temperature characteristic before
the correction. As will be clear from the figure, the control method
according to this embodiment enables the output frequency to be controlled
at an approximately constant level over a wide temperature range.
As has been described above, in the control method according to this
embodiment, a correction curve is obtained simply by measuring data at a
plurality of points, particularly at three points, for each oscillation
circuit 61, and the coefficients (temperature coefficients) of the
correction curve are obtained and stored in the storage circuit.
Therefore, the required storage capacity is minimized, and the number of
manhours is also reduced. Accordingly, it is possible to realize accurate
temperature compensation at reduced cost.
Although in this embodiment, data is measured at three points of
-20.degree. C., room temperature (10.degree. C. to 35.degree. C.) and
60.degree. C., it should be noted that any temperature points may be
selected. However, two points are preferably selected from among those
which are within temperature ranges of from -30.degree. C. to -10.degree.
C. and of from 50.degree. C. to 75.degree. C., where a particularly large
variation is observed.
In the control method according to this embodiment, a frequency deviation
curve (correction curve) is calculated from data measured at a plurality
of points, particularly at three points, and the coefficients (temperature
coefficients) of the correction curve are stored in the storage circuit.
Therefore, the required storage capacity is minimized, and the number of
manhours is also reduced. Accordingly, it is possible to construct an
oscillation circuit capable of accurate temperature compensation at
reduced cost.
Further, although it has heretofore been difficult with the conventional
technique to use a crystal resonator having a frequency-temperature
characteristic which differs from a reference characteristic to a
considerable extent, the present invention makes it possible to readily
use even a crystal resonator having a frequency-temperature characteristic
which differs from a reference characteristic to a considerable extent
because a correction curve for each crystal resonator is directly
obtained. Thus, the frequency control method is superior in flexibility to
the conventional technique.
Even when the crystal resonator alone has been replaced with another, the
correction curve can be readily corrected.
Next, still another method of correcting the frequency of a digital control
type oscillation circuit of a portable telephone according to the present
invention will be explained.
As shown in FIG. 3, variable-capacitance diodes vary in the slope K, and
the conventional practice is to select a variable-capacitance diode 42
having a slope close to a predetermined slope K and to store the slope K
in the memory 67 in advance. Accordingly, it takes a great deal of time to
select a proper variable-capacitance diode. Moreover, an error is
introduced into the control voltage (correction voltage) V.sub.i because
the slope of the control voltage-frequency deviation characteristic is not
accurate.
Further, the slopes K of the straight lines shown in FIG. 3 are not uniform
over the entire control voltage range. That is, the control
voltage-frequency deviation characteristic of each variable-capacitance
diode is given by an approximately linear expression. Accordingly, an
error arises if correction is made using a control voltage (correction
voltage ) V.sub.i which is given by the linear expression (1) with a slope
K different from that of the nonlinear voltage value portion. For example,
in the case of a crystal resonator 45 having a temperature characteristic
such as that shown by MIN in FIG. 9, the frequency correction results in
as shown in FIG. 17. That is, the largest error e.sub.1 is 0.6 ppm in
terms of the output frequency. In the case of a crystal resonator 45
having a temperature characteristic such as that shown by MAX in FIG. 9,
the frequency correction results in as shown in FIG. 18. That is, the
largest error e.sub.3 is 0.5 ppm in terms of the output frequency. These
errors, considered together with other error factors of the crystal
resonator 45, exceed 1 ppm and do not meet the requirements that the
frequency deviation shall be within .+-.1 ppm in the temperature range of
from -20.degree. C. to 60.degree. C., as specified by RCR-STD (RCR
Standards).
Therefore, in this embodiment, the control voltage to be applied to a
variable-capacitance diode used for temperature compensation is controlled
according to the control voltage-frequency deviation characteristic of the
diode which is divided into a plurality of control sections, thereby
improving the frequency accuracy.
The block arrangement of a digital temperature-compensated crystal
oscillator to which the frequency correcting method according to this
embodiment is applied, and the oscillation circuit, are the same as those
shown in FIGS. 1 and 2; therefore, description thereof is omitted.
FIG. 19 shows the characteristic of a variable-capacitance diode used in
the frequency correcting method according to the present invention. As
illustrated in the figure, the slope K of the variable-capacitance diode
42 is slightly different for the central portion than for both end
portions. For example, in the control section b, in which the frequency
deviation is from zero to .+-.4 ppm, K is 0.057, whereas, in the control
sections a and c, K is 0.06. It should be noted that the slope K is
represented by the above expression (5). A control voltage (correction
voltage) to be applied to the variable-capacitance diode 42 is calculated
according to the above expression (4) (V.sub.i =V.sub.0 -.DELTA.f.sub.i
.multidot.K), and the calculated control voltage is applied to the
variable-capacitance diode 42.
The crystal resonator 45 exhibits a temperature characteristic such as that
shown in FIG. 9, which is generally given by a cubic expression with
respect to temperature. The slope K of the variable-capacitance diode 42
and the temperature characteristic (cubic expression) of the crystal
resonator 45 are previously stored in the memory 67. The CPU 64 calculates
a frequency deviation from the temperature detected with the temperature
sensor 62 and the temperature characteristic of the crystal resonator 45.
Then, the CPU 64 calculates a control voltage by the expression (1) using
the slope K of the variable-capacitance diode 42 in the corresponding
control section among the three control sections a, b and c, and applies
the calculated control voltage to the variable-capacitance diode 42.
FIG. 20 shows the way in which the control voltage (V.sub.cont) is changed
relative to the temperature by the frequency correcting method according
to the present invention. In this case, a crystal resonator having a
temperature characteristic such as that shown by MIN in FIG. 9 is used.
Thus, the frequency deviation (error) can be kept within 0.1 ppm by
changing the control voltage relative to the temperature as shown in FIG.
20. FIG. 21 also shows the way in which the control voltage (V.sub.cont)
is changed relative to the temperature by the frequency correcting method
according to the present invention. In this case, a crystal resonator
having a temperature characteristic such as that shown by MAX in FIG. 9 is
used. Thus, the frequency deviation (error) can also be kept within 0.1
ppm by changing the control voltage relative to the temperature as shown
in FIGS. 20 and 21.
Although in the above-described embodiment the control voltage to be
applied to the variable-capacitance diode 42 is controlled according to
the control voltage-frequency deviation characteristic of the diode 42
which is divided into three control sections, if the number of control
sections is increased, the frequency control becomes even more accurate.
As has been described above, the control voltage-frequency deviation
characteristic of the variable-capacitance diode 42 is divided into three
control sections a, b and c, as shown in FIG. 19, and the slope K
(K=(V-V.sub.0)/.DELTA.f; see the above expression (5)) of the
variable-capacitance diode 42 is set for each control section on the basis
of measured values . Therefore, frequency deviations (errors) reduce in
all the control sections, and thus it is possible to effect even more
accurate frequency control than in the conventional practice.
Thus, according to the frequency correcting method, the control
voltage-frequency deviation characteristic of a variable-capacitance diode
used for temperature compensation is divided into a plurality of control
sections, and a slope K of the variable-capacitance diode is set for each
control section on the basis of measured values. Therefore, frequency
deviations (errors) reduce in all the control sections, and thus it is
possible to effect even more accurate frequency control than in the
conventional practice.
Further, according to the method of the present invention, frequency
correction can be realized simply by storing the necessary coefficients in
the memory using the conventional hardware as it is. Therefore, no extra
cost is required. Furthermore, because no feedback control is used, the
control operation is rapidly stabilized. Therefore, the present invention
is most suitable for use in portable telephones.
Top